1. Field of the Invention
The present invention relates to a direct-injection internal-combustion engine and to processes allowing such an engine to be used.
2. Description of the Prior Art
The state of the art can be illustrated by the following patents: EP-849,448; EP-589,178 corresponding to U.S. Pat. No. 5,363,820; JP-10,184,362; JP-10,184,363; U.S. Pat. No. 3,945,351; GB-2,134,181; DD-147,386.
The development of combustion systems for internal-combustion engines must meet requirements such as emission reduction, torque and specific power increase, combustion noise reduction, while remaining compatible with endurance criteria.
Engineers currently have a wide range of possible modifications allowing these objectives to be reached.
Engineers can change, for example, design or adjustment parameters, like the shape of all or part of the combustion chamber, such as the bowl, provided in the upper part of the piston, in order to facilitate air/fuel mixing.
Engineers can also change the compression ratio, certain parameters of the injection system (pressure, control law, etc.) and of the fuel jets (number of holes, diameter, etc.), the pressure charging adjustment, the EGR (exhaust gas recirculation) ratio or the intake temperature.
Unfortunately, it is increasingly difficult to improve one of these points without deteriorating another one.
A well-known example is the improvement of the following compromise: NOx emissions/particles emissions.
The problem is even more difficult to solve when endurance criteria have to be taken into account.
This is for example the case for the liquid fuel or the soot in the vicinity of the cylinder walls, whose presence, harmful to the lubricant stability, depends on the nappe angle of the injection nozzle, on the shape of the bowl or on the timing and on the injection pressure. Adjustment of these parameters unfortunately has a harmful effect on the combustion, leading to an emissions increase and/or to a reduction in the full load performances and/or to a noise increase.
Patents EP-849,448 or EP-589,178 notably relate to combustion chambers for internal-combustion engines. These combustion chambers are defined on one side by the upper wall of the piston, which comprises a conical or truncated shaped part referred to as projection pointing towards the cylinder head and having the same axis, at the center of a circular cup or bowl. The fuel is injected through a multijet nozzle in line with each cylinder, with a very wide nappe angle of the order of 140 to 160xc2x0. The angle at the top of the projection is suited to the nappe angle of the injection nozzle so that the fuel jets are injected substantially along the slopes of the projection, then diverted thereby and by the rounded shape of the cup towards the top of the cylinder, thus vaporizing without ever touching the walls of the cylinder and therefore without affecting its lubrication. Of course, the pistons have to be quite close to the, top dead center (TDC) thereof for this effect to be obtained. Only a limited latitude is therefore available for selection of the fuel injection times.
This latitude constitutes a drawback that is by no means insignificant since it is also well-known that early and late fuel injections afford many advantages. Thus, a pilot injection before the top dead center and the main injection allows the combustion noise to be reduced. Regeneration of the particle filters, which is initiated, according to a known technique, by raising the exhaust temperature, requires a fuel injection shortly before opening of the exhaust valves. A fuel injection during expansion or during the exhaust phase can also be useful to obtain favorable exhaust conditions allowing regeneration of the NOx traps.
Since combustion systems generally use injection nozzles with nappe angles ranging between 140xc2x0 and 160xc2x0, the adjustment range of these injections is reduced to limit problems of lubricant dilution by fuel or soot.
In order to overcome this limitation, the present invention is an engine providing combustion allowing substantially varied injection conditions.
The direct-injection internal-combustion engine according to the invention, comprises at least one cylinder with a cylinder head, a piston sliding in the cylinder, a gas intake and exhaust, a combustion chamber defined on one side by the upper face of the piston comprising a projection pointing towards the cylinder head and arranged at the center of a concave bowl, the combustion chamber comprising at least one injection nozzle for injecting fuel with a nappe angle less than or equal to 2Arctg2FCD, where CD is the diameter of the cylinder and F the distance between the point of origin of the jets from the injection nozzle and the position of the piston corresponding to a crankshaft angle of 50xc2x0 in relation to the top dead center (TDC).
Using an injection nozzle with a relatively narrow nappe angle offers a great latitude for selection of the fuel injection times while avoiding the drawbacks linked with the wide dispersion of the fuel injected, notably regarding wetting of the cylinder walls. This type of injection nozzle is well-suited to the combustion mode referred to as homogeneous combustion.
According to an embodiment, the angle at the top of the projection is selected substantially in accordance with the nappe angle of the injection nozzle and the bowl is so shaped that the injected fuel is diverted towards the cylinder head by following a concave shape of the projection towards the outside of the bowl and vaporized without reaching the walls of the cylinder, for any position of the piston up to xc2x130xc2x0 in relation to the top dead center.
The combined use of the injection nozzle with a relatively narrow nappe angle and of a bowl of particular shape allows conventional operation for a fuel injection in the neighborhood of the top dead center, but within a wider angle range than the range that is usually allowed.
The nappe angle of the injection nozzle is advantageously selected between 0xc2x0 and 120xc2x0.
The nappe angle of the injection nozzle is preferably selected between 40xc2x0 and 100xc2x0.
The angle at the top of the projection is selected greater than the nappe angle by a value ranging between 0xc2x0 and 30xc2x0.
The height of the projection is greater than at least 40%, preferably at least 60% of the depth of the bowl.
The fuel jets from the injection nozzle form an intersection angle of the order of 5xc2x0 with the flank of the projection.
According to a particular feature of the invention, the bowl has a bowl diameter BD, a bowl depth L, a first curvilinear part of radius R1, a second curvilinear part of radius R2, a bowl bottom diameter FD, a lateral wall of inclination a3 and a diametral section length Cb, and the dimensions of the bowl meet at least one of the following conditions:
the BD/L ratio is less than 6 and preferably less than 4;
the BD/R1 ratio is greater than 4;
the R2/R1 ratio ranges between 0.4 and 1;
the FD/BD ratio is less than 0.8;
the Cb/BD ratio is less than 2;
the angle of inclination a3 is less than 45xc2x0.
The angle of inclination is negative so that the bowl has a re-entering neck which limits an outlet section of the bowl.
The neck is arranged at a height that is less than the total height of the bowl.
The dimensions of the bowl meet one of the following conditions:
the GD/BD ratio is less than 1;
the L2/L ratio is less than 1;
the S/V ratio is less than 0.1 mmxe2x88x921.
The process according to the invention for improving the latitude of operation of a direct-injection internal-combustion engine comprises at least one cylinder, a piston sliding in this cylinder, a gas intake and exhaust, a combustion chamber defined on one side by the upper face of the piston comprising a projection pointing towards the cylinder head at the center of a bowl, the fuel is injected into each cylinder through an injection nozzle with a nappe angle that is less than or equal to 2Arctg2F2D, where CD is the diameter of the cylinder and F the distance between the point of origin of the fuel jets from the injection nozzle and the position of the piston corresponding to a crankshaft angle of 50xc2x0 in relation to the top dead center (TDC).
A piston in accordance with the invention is such that the angle at the top of the projection is substantially in accordance with the nappe angle of the injection nozzle, and a bowl whose wall is so shaped that the injected fuel is guided towards the outside thereof and vaporized without reaching the cylinder walls, for any position of the piston up to xc2x130xc2x0 in relation to the top dead center.
The invention avoids the presence of liquid fuel on the cylinder walls while affording variable engine performance and pollution control possibilities.
A process of the invention for improving the latitude of operation of a direct-injection internal-combustion engine comprises at least one cylinder, a piston sliding the at least one cylinder, a gas intake and exhaust, a combustion chamber defined on one side by an upper face of the piston comprising a projection pointing towards the cylinder head, comprises:
for low and average loads, injecting fuel into each cylinder through an injection nozzle with a nappe angle less than or equal to 2Arctg2FCD, where CD is the diameter of the cylinder and F is the distance between the point of origin of fuel jets from the injection nozzle and the position of the piston corresponding to a crankshaft angle of 50xc2x0 in relation to the top dead center (TDC) to provide a long mixing time of the fuel with the air admitted before combustion of this mixture,
for high loads, injecting the fuel through the same injection nozzle with the same nappe angle, so as to obtain stratified mixing of the fuel with air and a progressive combustion.
At full load and at high loads, the engine according to the invention allows good carry-over of the vapor fuel thus favoring mixing thereof with the air in order to obtain a good combustion rate and high air/fuel ratios as a result of a good use of the air admitted. At partial loads, a reduced nappe angle allows a wide injection system adjustment latitude without the cylinder walls being wetted.